In conventional systems is known to provide capacitive sensor devices for detecting an approaching object or a contact, respectively, by means of which an approach of an object towards a sensor electrode or a movement of an object relative to the sensor electrode may be measured in contactless fashion. The measurement signal is interpreted and the result may be used to trigger functions, for example switching functions of an electric device, in particular of an electric hand-held device.
Conventional capacitive sensor systems comprising one or several sensor electrodes, however, have the disadvantage that during a movement of an object, for example of a finger, relative to the sensor electrode the measurement values during constant distance of the object to the sensor electrode may differ in absolute value.
For example, when the measurement values of a sensor electrode of a capacitive sensor system, which is widespread and flat as compared to a finger, are observed during a movement of a finger in the direction of X with constant Y and Z, wherein Z represents the distance of the finger from the electrode plane, then the measurement values initially increase, have a maximum approximately in the center of the electrode plane and then decrease. During a sufficiently quick movement of the finger in the direction of X relative to the electrode plane, the measurement signal looks similar to the measurement signal of a contacting movement, i.e. of the movement of a finger in the direction of Z. This means that a change of the measurement value of the sensor electrode may not conclusively be related to a change of the distance of the finger from the sensor electrode (Z distance). The variation of the measurement values also may have other reasons. Particularly for the contact detection (at a distance Z=0), in consequence of the varied dependencies of the measurement value, i.e. the measurement value not only depends on the distance of the finger from the sensor electrode, the interpretation of the signal curves partially is so much complicated that certain functionalities, for example “drag and drop”, may not be ensured reliably.
A similar problem also occurs in strip-type sensor electrodes SE known in prior art, as for example shown with reference to FIG. 1. When a finger F at a constant distance Z between the finger F and the sensor electrode SE moves across the sensor electrode SE in the direction of X, then the measured sensor value changes as shown in the signal curve in FIG. 1. For example, as shown in FIG. 1, in a strip electrode of a length of 10 cm (X=−5 cm to X=+5 cm) the measured sensor value will be u (X=0)>u (X≠0) because the capacity between the sensor electrode SE and the finger F is greatest in the center of the strip, i.e. at X=0, while the capacity between the sensor electrode SE and the finger F is smallest at the edge regions. Theoretically, there is information in this variation of the measurement values; however, it may not be directly related to the movement of the finger F in a certain direction.
In conventional systems, it is attempted to take into account this non-homogeneous behavior in the signal processing, i.e. in the processing of the measurement signal of the sensor electrode SE, by additionally taking into account information of other sensors. For example, the Z-value may be determined from weighted sensor values of several adjacent sensor electrodes. However, this solution only works satisfyingly for Z>>0 when the finger clearly is inside the detection zone of all electrodes. However, this solution is not appropriate for contact detection (Z=0), since on the basis of the ambiguity of the measurement values, i.e. during a contact the measurement value not only depends on the distance Z of the finger from the sensor electrode SE, the interpretation of the signal curves in part is complicated that considerably that certain functionality may not be ensured reliably.